For more than 100 years, chemicals obtained as by-products in the primary processing of coal to coke have been the main source of aromatic compounds used as intermediates in the synthesis of dyes, drugs, antiseptics, solvents and other products. Although some aromatic hydrocarbons, such as toluene and xylene, are now obtained largely from petroleum refineries, the main source of many others is still the coke oven.
Unfortunately, in addition to producing important products by depolymerization of coal, the refineries and by-product coke ovens, by their mode of operation, also unavoidably and largely uncontrollably discharge, to the environment, enormous quantities of heat and materials hazardous to the environment and to human health. Operation of by-product coke ovens requires high temperatures, which favor the formation of high molecular weight "ring" compounds, of limited practical value, such as anthracenes, fluoranthracene, pyrene, benzanthracene, dibenzanthracene, benzpyrene, and the like, and inhibit the formation of more useful, low molecular weight compounds such as phenols, benzoic acid, phthalic acid, other carboxylic acids, and the benzylic alcohols containing simple aromatic ring systems. As a result, it would be advantageous if coal could be subjected to low temperature depolymerization processes to yield compounds such as the aforementioned, more useful ones.
Enzymes are known that catalyze the depolymerization of various biopolymers. Further, enzymatically catalyzed reactions occur typically at low temperatures between about 20.degree. and about 70.degree. C., and, hence, do not have the potential of stressing the environment in ways that are associated with traditional coal processing. Heretofore, however, no enzyme has been known to catalyze depolymerization of coal.
Two reasons for the absence of recognition in the art of coal depolymerization-catalyzing enzymes are that enzymes function best in aqueous environments and that enzymes require soluble substrates in order to catalyze reactions at high rates. Thus, heretofore, the insoluble nature of coal has presented the potential enzyme depolymerization catalyst with an intractable substrate such that, even with increases in surface area furnished by grinding coal into small particles, reaction rates indicative of enzymatic catalysis could not have been observed, even if enzymatically catalyzed reactions were occurring. The discovery of the present invention, of high molecular weight coal substrates that are soluble in aqueous solution under conditions suitable for enzymatic activity, has made possible the discovery of the invention, that certain enzymes catalyze coal depolymerization.